The present invention relates to an improved actuator unit suitable for use with a performance operator member, such as a key of a piano, a keyboard musical instrument provided with such an actuator unit, and an actuator unit assembly composed of a plurality of the actuator units.
In the so-called automatic player pianos, keys of a keyboard musical instrument, such as a piano, are automatically operated to execute an unattended musical performance. Such automatic player pianos are arranged to selectively drive a plurality of key drive devices, provided in corresponding relation to the keys, on the basis of automatic performance information to be performed. Japanese Patent Application Laid-open Publication No. HEI-5-28195 discloses an example of a conventionally-known key drive device (i.e., actuator) for an automatic player piano. According to the disclosure in the No. HEI-5-28195 publication, the actuator in the form of an electromagnetic solenoid is disposed beneath a back portion of each key, and unattended (or automatic) key depressing operation is carried out by a movable member of the solenoid pushing up the back portion of the corresponding key.
In addition to driving the keys via such the actuators (electromagnetic solenoids) on the basis of automatic performance information, it has recently become a common practice to impart a desired touch feeling (or touch) to key operation of a human player by variably controlling a force F for driving the actuator (electromagnetic solenoid) (i.e., “force sense control”). Such “sense of force control” is performed, for example, by using a suitable position sensor to detect displacement of the movable member of the solenoid, key itself or the like and then controlling driving of the actuator, on the basis of the detected displacement (position information), to thereby impart a desired sense of force or reactive force to the key. It has been known that the force F for driving the actuator can be obtained by the following equation of motion:F=MX″+PX′+KX  Mathematical Expression (1),where X represents position information, X′ represents velocity information, X″ represents acceleration information, M represents mass, P represents an adhesion coefficient, and K a spring coefficient.
Further, in the case where the force sense control is performed using the actuator (electromagnetic solenoid) of the above-mentioned type or the solenoid is merely subjected to feedback control, it has been known to employ a plunger sensor section which includes an optical sensor provided on the underside of the plunger, and where an operating state of the plunger is detected on the basis of reflected light from a surface opposed to the underside of the plunger (for example, Japanese Patent Application Laid-open Publication No. 2001-34261).
With the conventional actuator for automatic player pianos, as represented by the one disclosed in the No. HEI-5-28195 or No. 2001-34261 publication, only the position is detected, for example, by a position sensor without the necessary physical amounts, i.e. position, velocity and acceleration, being detected separately, and then the velocity and acceleration are determined by differentiation of the thus-detected position. In the case where the force sense control is to be performed using the actuator, it has been conventional to generate a detection signal (control signal) for controlling the actuator driving force F, by obtaining the parameters MX″, PX′ and KX in Mathematical Expression (1) above on the basis of the sensor-detected position information and velocity and acceleration information determined through differentiation of the position information.
However, calculating the velocity and acceleration information through differentiation of the position information as mentioned above would present the inconvenience that the calculated values tend to include error components. Further, if the acceleration information is calculated through differentiation of the position information, there arises a need to perform an extra filtering process in order to remove noise components included in the value obtained through the differentiation operation, which would thus result in significant time delays. Deterioration in signal quality due to the errors in the detection signals and time delays is undesirable in the driving control of the actuator, and particularly the touch impartment control, because it would cause lowering of control accuracy.
If separate sensors are provided, in the above-discussed conventional actuator for automatic player pianos, for detecting different types of physical amounts, detection signals of the different types of physical amounts could have improved quality. However, with the conventionally-known sensor construction, the provision of the separate sensors presents the problem that the actuator unit tends to be considerably complicated in structure and considerably increase in size. For example, according to one example of a feedback sensor for an actuator disclosed in Japanese Patent Application Laid-open Publication No. 2001-34261 which corresponds to U.S. Pat. No. 6,420,642, a current position of the plunger is detected on the basis of variation in the amount of reflected light corresponding to variation in the distance between the solenoid and the sensor; in this case, however, the actuator unit has to have an overall height that is at least equal to a combination of a height corresponding to a stroke amount of the plunger and the height of the sensor itself Further, in the case where a gray scale is detected via a photo-interrupter, as discussed as a related prior technique in the No. 2001-34261 publication, it is difficult to downsize the sensor, because the gray scale projects from a side surface of the plunger and the photo-interrupter is positioned in such a way as to face opposite surfaces of the gray scale.